Oldest fossil of complex brain identified

10 October 2012

Illustration of the head of the 520-million-year-old animal Fuxuianhuia protensa on the left and a hermit crab on the right showing the similarities in brain complexities.

It may have lived more than half a billion years ago, but this ancient animal Fuxianhuia protensa had a brain just as complex as today's lobsters and crabs, scientists report in the journal Nature today.

Fossil of the whole animal, Fuxuianhuia protensa, from the Chengjiang site in China. It was 11cm long.

Fuxianhuia protensa was an early arthropod (animal with an exoskeleton, segmented body and jointed legs) and the 11cm-long predator had eyes on stalks. It lived in the Cambrian Period (542-488 million years ago).

Its 520-million-year-old fossilised brain was identified by an international team, including the Natural History Museum's Dr Xiaoya Ma and Dr Greg Edgecombe.

The fossil is from the Chengjiang site in southern China, and is extremely well-preserved. It shows preserved brain as dark brown patches and is the oldest fossil example of a complex brain in early animals.

The team used the Museum's scanning electron microscope (SEM) to look at the surface structure of the brain material and an energy-dispersive X-ray spectroscope to see how the details of the brain and optic tracts were preserved. They found that the brain was made up of 3 sections that were connected to the head segments. And nerve fibre tracts ran from the brain to the eyes and antennae.

'We were surprised by the degree of complexity, which was more comparable to that seen in living crustaceans and insects,' said Edgecombe.

'The obvious comparisons are with crustaceans like lobsters or prawns because they also have movable eye stalks. The eye stalks have elaborate concentrations of neural tissue inside them, and the brain has 3 segments that are fused in front of the mouth.'

Close-up of the preserved brain and optic nerves shown as dark brown patches.

The team says the animal could rotate its eye stalks over quite a wide arc, making it able to see in different directions. 'It must have had quite acute vision,' Edgecombe says. 'This is consistent with its relatively sophisticated brain because we would expect big, movable eyes to be associated with quite sophisticated neural wiring'.

It is thought that Fuxianhuia branched off from the arthropod evolutionary tree before the common ancestor of today's arthropods appeared. 'The common ancestor of this diverse group [arthropods] may have also had elaborate brains similar to those of lobsters or flies, rather than to the simpler brains of, say, brine shrimp or fairy shrimp,' said Ma.

Chengjiang fossil site

It's extremely rare to find fossilised soft tissue. That is, except for the site where Fuxianhuia was uncovered. 'Chengjiang is one of the places where soft anatomy of many different kinds of animals is preserved in exceptional detail,' said Edgecombe.

The site seems to have had unique conditions that allowed this preservation, Edgecombe explains. 'The keys to preserving soft anatomy in Cambrian fossils seems to be low oxygen conditions that inhibit bacterial activity soon after the animal dies, and then rapid burial of the carcass in fine-grained sediment.'

Why did Fuxianhuia die out?

So, why did an animal with such a complex brain die out? Ma gives her view. 'Natural selection is about adapting to changes over evolutionary history and doesn't necessarily depend on how complex the organism is. Very complex animals are often highly adapted to a certain environment, and if it changes dramatically, they can die out. Simple organisms like algae and bacteria often survived these kinds of events.

'Also, complex organs can be very costly. If there is no need for the complex structures, evolving a simpler structure will help the animal survive. For example, some cave animals become blind or have reduced visual organs. '

Finding its true relations

Edgecombe says the research raised some new questions. 'For example, is Fuxianhuia really as deeply nested in the arthropod stem-group as we have thought was the case, or could it be more closely related to some living arthropods?

'Is the similarity of the brain and eye anatomy to that of living crustaceans and insects a result of common ancestry or convergent evolution (where organisms that are not closely related evolve similar structures independently from each other), perhaps as a result of being a highly visual arthropod?

'The great thing is that we can address these questions with new data from a source that we really haven’t had much to work with before – fossil brains.'